While the homopolymer of vinylidene chloride is generally insoluble at room temperature in conventional organic solvents, e.g., in pure tetrahydrofuran, useful materials, soluble in solvents such as mixtures of tetrahydrofuran and toluene at room temperature, can be made by copolymerization of vinylidene chloride with lesser quantities of other monomers such as acrylonitrile, methacrylonitrile, methyl acrylate (other C.sub.2 -C.sub.13 alcohol esters of acrylic or methacrylic acid, e.g., methyl methacrylate), vinylidene cyanide, acrylic acid, itaconic acid, and chloroacrylonitrile, vinyl chloride, vinyl bromide, vinylidene bromide and the like.
These materials find particular use in the cellophane industry where a very thin (usually 0.05 mil) coating of so-called "soluble" vinylidene chloride polymers is deposited from solvent such as the solvent combinations mentioned above, on one or both sides of cellophane. This thin coating serves several purposes in that it causes the cellophane film to be a good gas or water-vapor barrier film for packaging purposes where either a loss of or gain in gas or water content of the product would render the product less attractive for sale. In addition, the coating promotes retention of water (which acts as a plasticizer usually along with some glycol or glycerine) in the cellophane, thus helping to prevent brittle fracture of the film. Finally, such coatings provide a heat-sealable outer coating where cellophane, by its very nature, cannot be heat-sealed.
For cellophane manufacturers concerned with applying such thin coatings to cellophane, the solubility, heat-seal temperature, flexibility, and water-vapor transmission rate (WVTR) are particularly important properties. Usually the desired properties include as low a heat-seal temperature and WVTR as possible coupled with low temperature solubility in inexpensive solvents or solvent combinations.
Since the WVTR of a vinylidene chloride interpolymer is directly related to the mole percent of vinylidene chloride in the polymer, as the most important factor in the manufacture of such resins, it is advantageous to get the mole percent of vinylidene chloride as high as possible consistent with solubility in the desired workable solvent systems. Since high vinylidene chloride content means strong crystalline forces in the polymer, these two factors are diametrically opposed, i.e., high vinylidene chloride content, highly crystalline polymers being the least soluble but offering the lowest WVTR.
As the mole percent of vinylidene chloride is raised in a polymer series, it is obvious that a percentage range is reached where the polymers rapidly change from ones which are permanently amorphous or which crystallize only slowly and incompletely in coatings to ones which are highly crystalline and difficult to dissolve. Consequently, careful control over the polymer composition during polymerization is needed to avoid the formation of a higher vinylidene chloride content than intended, since such could result in polymers unsuitable for coating purposes due to insolubility or premature crystallization in solution. The tendency for such polymers to be insoluble or prematurely crystallize in solution may be measured by light transmission measurements of a 15 to 20 percent lacquer solution in, for example, a solvent mixture of 65 percent tetrahydrofuran and 35 percent toluene, after aging the solution at 25.degree. C. for 24 hours, against a reference of the pure solvent mixture. Careful control of the percent light transmission, hereinafter called "haze", is essential for coating use of these polymers, especially those having a high mole percent vinylidene chloride, since the tiny insoluble crystals which cause the haze by incomplete dissolution also serve as nuclei for overall crystallization once the interpolymer has been deposited as a thin coating on a substrate. The rate of crystallization in a coating, which is affected by the number of nuclei present, is very important for commercial operations since a rate of crystallization which is too slow will result in blocking of the coated film (which is typically wound in large rolls long before crystallization is complete).
It is desirable, by judicious choice of comonomers and proper reaction methods to optimize the heat-seal temperature, WVTR-solubility relationship, and flexibility and approach an ideal composition for coating use. While much work has been done in the past to find these combinations of comonomers to use for copolymerization, and much effort has also been expended to find the best method of copolymerizing these comonomers to give the best combination of heat-seal temperature, flexibility, WVTR, and solubility, a combination of monomers and method for the polymerization thereof has now been discovered which is superior to anything known heretofore for attainment of optimum barrier, heat-seal and solubility properties in the absence of the use of a major proportion of acrylonitrile or methacrylonitrile.